Generally, in attaching various parts to a lightweight base plate adapted to be incorporated into an electric apparatus such as a television set or stereophonic unit or to the body of a mechanical apparatus, such base plate or body is normally mounted on a pallet and is conveyed. The pallet is stopped at an assembly station, necessary parts are attached manually or with robots to the base plate or body mounted on the pallet, then the pallet carrying the base plate or body is conveyed to the next assembly station, and then similar assembling processes are repeated while the pallet is conveyed along pallet guide rails.
The pallet conveying passages employed in such an assembling process are formed in a closed passage or loop consisting of a conveying side along which the pallet is moved and stopped for assembling works and a return side along which empty pallets are returned, which sides are connected at the respective opposite ends by lifters, traversers and/or turntables.
A plurality of assembling stations are arranged along the pallet conveying passage. Automatic assembly robots or assemblers are positioned at the assembling stations. The pallet has to be stopped at each assembling station until the assembling work is completed. Particularly in robotizing the automatic assembling work, the positioning accuracy in stopping a pallet carrying the body is as severe as 50 .mu.or below. Further, the shock on the pallet during starting and during stopping must be mitigated to the minimum possible degree. Still further, the pallet conveying system has to be provided with both a collision preventing function to prevent the collision of the following pallet against the preceding pallet when the preceding pallet is stopping and a pallet reserving function. The collision preventing and pallet reserving functions are also necessary for the pallet return passage.
The conventional pallet conveying system employed in incorporating parts to a lightweight base plate or body execute pallet conveyance advantageously by means of frictional driving, which is operated simply for starting and stopping the pallet and is capable of causing less shock on the pallet during starting and stopping. For example, as described in Japanese Patent Publication No. 45568/77, the frictional driving system includes a long rotary shaft disposed along the pallet guiding rails and a driven roller attached to the underside of the pallet so as to be controlled in respect of the swiveling angle and to be pressed against the circumference of the rotary shaft. This arrangement is capable of easily starting and stopping the pallet by simply controlling the inclination of the driven roller relative to the rotary shaft and, since the driven roller is driven by the rotary shaft through frictional engagement, such system is advantageous in that less shock is imposed on the pallet during starting and stopping. On the other hand, this frictional engagement has disadvantages in that the accuracy of stopping the pallet at a fixed position is reduced, the initial starting motion is delayed, the accuracy of stopping the pallet at a fixed position is further reduced due to displacement of the stopping pallet from the correct stopping position caused by collision of the following pallet against the preceding pallet from behind owing to the incapability of absorbing the inertia of the following pallet, and thus the robotization of an automatic assembling system is impossible. Furthermore, the pallet of the abovementioned patent publication has a front lever guiding cam extending rearward from the rear end thereof and is not provided with a rear lever which is similar to the front lever, and therefore collision prevention and reservation of the pallet in the return passage are impossible.
This invention has been made to eliminate those disadvantages of the conventional pallet conveying system and to provide a pallet conveying system provided with new functions such as block control of the pallet, efficient supply of conveyance controlling power and improved assembling efficiency through provision of increased number of automatic assembling stations. An embodiment of the present invention will be described hereunder with reference to accompanying drawings.